Cerebral aneurysm treatment:modern neurovascular techniques

Bowen Jiang,1 Michelle Paff,2 Geoffrey P Colby,1 Alexander L Coon,1 Li-Mei Lin2

To cite: Jiang B, Paff M,Colby GP, et al. Cerebralaneurysm treatment:modern neurovasculartechniques. Stroke andVascular Neurology 2016;00:e000027. doi:10.1136/svn-2016-000027

Received 19 June 2016Revised 26 July 2016Accepted 28 July 2016

1Department ofNeurosurgery, Johns HopkinsUniversity School ofMedicine, The Johns HopkinsHospital, Baltimore,Maryland, USA2Department ofNeurosurgery, University ofCalifornia, Irvine School ofMedicine, UC Irvine MedicalCenter, Orange, California,USA

Correspondence toDr Li-Mei Lin;limei.lin@uci.edu

ABSTRACTEndovascular treatment of cerebral aneurysm continuesto evolve with the development of new technologies.This review provides an overview of the recent majorinnovations in the neurointerventional space in recentyears.

HISTORY OF ENDOVASCULAR TREATMENTThe neurosurgical treatment of intracranialaneurysms dates back to 1937, when Dandy1

described microsurgical obliteration ofa posterior communicating artery aneurysm.For decades, microsurgical clipping remainedthe gold standard and foremost modalityfor treatment of cerebral aneurysms.Endovascular therapies emerged in the 1990swith the advent of the Guglielmi detachablecoil system. This system established neuroin-tervention as a new field, with multiple rando-mised clinical trials demonstrating the efficacyand safety of coil embolisation.2 3 Despitethese early favourable results, postcoilinganeurysm recanalisation remained a challengein the field. For instance, Raymond et al’s4

experience with 501 cerebral aneurysmstreated with endovascular coiling demon-strated that complete angiographic occlusionrate was approximately only 38% at 1-yearfollow-up. The data were further corroboratedby Gory and Turjman,5 whose prospective,multicentre European study of 404 aneurysmstreated with Nexus detachable coils(ev3-Covidien, Irvine, California, USA),showed 22% neck remnant and 30% aneurys-mal remnant, with a 17.7% recanalisation rateand 21.6% thrombosis rate at 13 monthsangiographic follow-up.To address these shortcomings, the neuroen-

dovascular space quickly experienced significanttechnological advancements aimed at improv-ing the different properties of a coil, includingcoil lengths, shapes, softness and detachmentzones. These developments have translated toimprovements in clinical outcomes of cerebralaneurysms treated with coiling. For example,HydroCoils (MicroVention, Tustin, California,USA) have allowed for treatment of more

complex aneurysmal configurations with areduction in recurrence rates compared withbare-platinum coils.6 7 Additionally, new neu-roendovascular devices such as intracranialstents and balloons were developed toaugment coil embolisation. In balloon-assisted coil embolisation, typically a compli-ant balloon is positioned across the neck ofthe aneurysm to provide a scaffold protectingthe parent artery while coils are deployedthrough a microcatheter into the aneurysmsac. This reduces the risk of coil prolapseinto the parent vessel and can also provideimmediate protection in an event of intra-procedural aneurysm rupture during thecoiling due to the ability to achieve endovas-cular proximal control with balloon inflation.Comparatively, stent-assisted coil embolisa-tion was developed to improve the occlusionrate and coil packing density of wide-neckedas well as large and giant aneurysms. Duringthis process, the stent (similar to a balloon)is positioned across the aneurysm neck, pro-viding a scaffold to protect the parent artery.This minimises coil loop prolapse and allowsfor higher density coil packing, leading to areduction in recurrence rates and higherrates of angiographic occlusion.8–10

Despite these technological advancementsin coil embolisation, aneurysms with largediameters (>10 mm), wide necks, unfavour-able dome-to-neck ratios (<2) and fusiformconfiguration remain therapeutic challengesand dilemmas, with as high as >20% pooroutcome (aneurysm recurrence or treat-ment-related morbidity/mortality) associatedwith the management of large/giant aneur-ysms.11 12 To meet these challenges, deviceinnovations have ushered in new therapeuticconcepts, including flow diversion and intra-saccular flow disruption.

FLOW DIVERSION REVOLUTIONThe concept of flow diversion serendipit-ously stemmed from the lessons garneredfrom stent-assisted coil embolisation. Whenresearch studies demonstrated that denser

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coil packing with less coil prolapse into the parent vesselcorrelated with improved clinical and radiographic out-comes, fluid dynamic analysis revealed that endovascularstents actually accelerated aneurysmal thrombosis andalteration in blood flow into the aneurysm from theparent vessel.9 13 14 Flow diversion is fundamentallybased on two concepts illustrated in figure 1: (1) theplacement of a high-mesh density stent in the parentvessel disrupts blood flow into the aneurysm and (2) thestent provides a scaffold for which endothelium cangrow, subsequently isolating the aneurysm from theparent circulation.15 Flow diversion thus allows forprogressive intra-aneurysmal thrombosis over time withsubsequent radiographic obliteration of the aneurysm.The advantage of endoluminal flow diversion over endo-saccular coiling is the ability to treat the weakenedabnormal arterial wall by providing a scaffold for neoen-dothelialisation to occur. This neoendothelialisationprocess results in a durable occlusion of the aneurysmthat often provides a curative outcome as compared withthe known recurrence associated with coiling. Figure 2demonstrates a case example of a large left-sided cavern-ous internal carotid artery (ICA) aneurysm that wastreated with a flow diverter deployed within the parentICA and evidence of complete aneurysm occlusion onthe 6-month follow-up angiography. Additionally, withthe endoluminal approach, the aneurysm sac does notneed to be accessed during the treatment, which subse-quently eliminates the risk of intraprocedural aneurysmrupture inherent with endosaccular coiling.

The first introduction of flow diversion into theneuroendovascular space was in 2007, with the inventionof the Pipeline Embolization Device (PED; MedtronicNeurovascular, Irvine, California, USA).16 Since then,a plethora of flow diverters have entered the neurointer-ventional field including the Silk flow diverter (BaltExtrusion, Montmorency, France), Flow-RedirectionEndoluminal Device (FRED; MicroVention, Tustin,California, USA), Surpass (Stryker Neurovascular,Freemont, California, USA), Tubridge (MicroPortMedical, Shanghai, China) and p64 Flow ModulationDevice (Phenox, Bochum, Germany). Most of these flowdiverters are commercially available in Europe andSouth America. In the USA, however, PED is the onlyavailable flow diverter after its approval by the Food andDrug Administration (FDA) in 2011.17

Pipeline Embolization DeviceThe PED is a braided mesh designed with 48 strandsconsisting of 25% platinum–tungsten and 75% cobalt–chromium–nickel alloy. When fully deployed, the PEDprovides 35% metallic surface-area coverage with poresize of 0.02–0.05 mm2 at nominal vessel diameter.15 ThePED is available in sizes ranging from 2.5 to 5.0 mm indiameter (in 0.25 mm increments) and 10–35 mm inlength (in 2 mm increments from 10 to 20 mm, and5 mm increments from 20 to 35 mm). The PED deliverysystem comes attached to a 0.016″ diameter stainlesssteel delivery wire, with the segment where it is mounted

Figure 1 Flow diversion concept: placement of a high-mesh density stent (flow diverter) in the parent vessel disrupts blood flow

into the aneurysm (A and B), allowing for progressive intra-aneurysmal thrombosis over time with subsequent obliteration of the

aneurysm (C and D). Additionally, the flow diverter provides a scaffold for neoendothelialisation, which treats the weakened

abnormal arterial wall and isolates the aneurysm from the parent circulation resulting in durable occlusion of the aneurysm (E).

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being 0.008″ thick. It is delivered via a 0.027″ deliverymicrocatheter, such as the Marksman (MedtronicNeurovascular, Irvine, California, USA) or a similar sizemicrocatheter. The second-generation PED, namedPipeline Flex (PED Flex), received the European CEmark of approval in March 2014 and subsequentlyreceived FDA approval in February 2015. Its designincludes numerous delivery system changes to enhancedevice opening and provide additional safety with aresheathing feature.18 The PED’s on-label usage as dic-tated by the FDA is limited to the treatment of large orgiant (≥10 mm) wide-necked intracranial aneurysmsfrom the petrous to the superior hypophyseal segmentsof the ICA.17

Clinical experience worldwide with PED has demon-strated the effectiveness, durability, safety and cost-effect-iveness of endovascular endoluminal reconstructionprimarily in large and giant aneurysms. Initial reportsfrom Buenos Aires and Budapest series demonstratedcomplete radiographic occlusion rates in the 90–93%range at 6 months follow-up angiogram.19 20 Thesestudies paved the way to the PED for the IntracranialTreatment of Aneurysms Trial (PITA), which showed93.3% angiographic occlusion rates at 6 months (28 of30 aneurysms), with 6.5% of patients having majorstroke or neurological complication (2 of 31 patients).16

Similarly, the PED for Uncoilable or Failed Aneurysms(PUFs) multicentre clinical trial that led to the FDA pre-market approval of the PED in 2011, demonstrated in108 patients across 10 prospective centres high rates ofprimary efficacy (81.8% of aneurysm with completeocclusion at 6 months) with low complication rate (5.6%of patients with major strokes or neurological injury at180 days).21 Taken together, all of the trials demon-strated relatively high rates of aneurysm occlusion (81.8–93.3%) with low rates of major morbidity and mortality(0–6.5%).16 19–21

Other subsequent studies for the on-label usage ofPED have corroborated the trial data at the institutionallevel, with similar rates of radiographic success with lowrates of postprocedural morbidity and mortality.22

Studies have further demonstrated the cost-effectivenessof using PED as advantageous over traditional

stent-assisted coiling of large anterior circulation aneur-ysms (27.1% cost reduction per millimetre of aneurysmtreated in the PED group compared with stent-assistedcoiling).23 Treatment of large and giant proximal ICAaneurysms using PED also requires less radiation, lessfluoroscopy time and less contrast administration thanstandard coiling techniques, with an average radiationdose of 2840 mGy with PED treatment compared with4010 mGy with traditional coiling techniques.24

Although PED is FDA approved for large and giantintracranial aneurysms, these aneurysms only encompassa small fraction of all intracranial aneurysms. In contrast,over 80% of all cerebral aneurysms found in the generalpopulation are <10 mm in size.25 26 There is considerabledocumentation in the literature that the majority of rup-tured aneurysms are smaller than 10 mm.27 Treatment ofsmall aneurysms (<10 mm) with flow diverters may beadvantageous over traditional endosaccular modalities.Lin and colleagues retrospectively reviewed a prospective,single-centre aneurysm database to identify 41 patientswith 44 PED cases. In this series, mortality was reportedin 2.3% while by 6 months post-PED implantation, angio-graphic success was observed in 80%.28 This experiencewith PED treatment for small ICA aneurysms is compar-able to the few published reports on this experience, withmortality range of 0% observed by Chan et al,29 and 11%observed by Lubicz et al,30 which is comparable with thePED experience for large and giant aneurysms withreported mortality rates up to 5.5%.A wealth of experience has been garnered in the neu-

roendovascular space with the PED since its introduc-tion, such that treatment of large and giant ICAaneurysms with flow diversion is now a widely acceptedstandard approach. As a result of the success with PEDtreatment of these complex aneurysms, there is also anexpansion of interest and data supporting the safety aswell as efficacy of PED treatment in an ‘off-label’manner with small anterior circulation aneurysms aswell as select posterior circulation aneurysms.31–33

Surpass flow diverterThe Surpass flow diverter is similar to the PED in designconsisting of a braided mesh constructed of cobalt–

Figure 2 Flow diversion treatment of a large left cavernous aneurysm with complete occlusion of the aneurysm demonstrated at

6-month follow-up angiogram.

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chromium alloy intertwined with platinum–tungstenwires for visibility. Compared with PED, the Surpass hasa lower porosity and is designed to maintain constantpore density over various diameters of the device.Additionally, the Surpass flow diverter exists in fewersizes with only 3, 4 and 5 mm diameter options. The 3and 4 mm devices have 72 wires, and the 5 mm device has96 wires. Unlike the PED, the Surpass device is preloadedat the distal end of a microcatheter delivery system thatconsists of a 0.040″ inner diameter (ID) delivery mico-catheter and pusher. The entire Surpass delivery system isadvanced over a 0.014″ microwire inserted through thepusher. The implant is deployed by a combination ofadvancing the pusher and unsheathing themicrocatheter.34

Wakhloo et al35 reported the largest study on safetyand efficacy of Surpass, with a prospective, multicentre,non-randomised, single-arm design of 165 patients with190 intracranial aneurysms of the anterior and posteriorcirculations enrolled from 24 centres. Successful deploy-ment of the Surpass flow diverter was observed in 98%of the aneurysms. Follow-up angiography available in158 (86.8%) intracranial aneurysms showed 100% occlu-sion in 75% of the cases at radiographic follow-ups.Permanent neurological morbidity and mortality were6% and 2.7%, respectively. Morbidity occurred in 4%and 7.4% of patients treated for aneurysms of the anter-ior and posterior circulation, respectively. These datademonstrate that the clinical safety profile is similar tothat of stent-assisted coil embolisation and PED treat-ment with high rates of radiographic occlusion.35

Colby and colleagues reported the initial NorthAmerican experience with Surpass. In this study, 20patients with ICA aneurysms ≥10 mm with ≥4 mm neckwere treated as part of the Surpass IntraCranialAneurysm Embolization System Pivotal Trial (the SCENTtrial; Stryker).36 The Surpass device was implanted in19/20 (95%) cases. Of 19 cases, a single device was usedin 18 cases (95%) and two devices in only 1 case (5%).Balloon angioplasty was performed in 8/19 cases (42%).Complete aneurysm neck coverage and adequate vesselwall apposition was obtained in all 19 cases withoutacute morbidity or mortality. An advantage of Surpassflow diverter is the Surpass delivery system that allows fora torque-free, over-the-wire deployment of the device.36

Additionally, this system provides the safety of maintain-ing a continuous endoluminal wire access that isuncoupled from the device deployment.

Other flow divertersSilkThe Silk is a self-expandable stent consisting of a tightlywoven structure, supplied with a soft microcatheter andhas resheathing and relocation abilities. For the majorityof studies in the literature, Silk is used for large andgiant neck fusiform aneurysms with a wide neck(dome-to-neck ratio of <2 mm or neck >4 mm). Straussand Maimon37 retrospectively reviewed patient data

from 2008 to 2013 and identified 60 patients with 67aneurysms (15 posterior circulation, 52 anterior circula-tion). They concluded a ‘good’ angiographic result in88% or 53/60 aneurysms, with a good outcome definedas complete or near-complete angiographic occlusion at15 months. The group identified aneurysm size as a posi-tive predictor of complication rate (0% in smaller aneur-ysms, 16.7% in large aneurysms and 42.9% in giantaneurysms). Giant aneurysms of the posterior circulationhad unfavourable results, with 57% complication ratedue to brainstem ischaemia secondary to perforatorocclusion.37 These data are further corroborated byLubicz et al,38 who reported a 11% delayed complicationrate with overall neurological morbidity of 5.5%;however, all complications were reported with the first-generation Silk device. In an eight-centre study inCanada by Shankar and colleagues, perioperative mor-bidity and mortality were 8.7% and 2.2%, respectively. Atthe last available follow-up, 83.1% of the aneurysms wereeither completely or near completely occluded, with therate of complications was higher for fusiform aneurysms(p<0.001).39

Taken together, these data show a trend towards betteroutcomes for smaller and anterior circulation aneurysmsand overall consistent radiographic outcomes for flowdiversion of large and/or fusiform aneurysms with theSilk device.

Flow-Redirection Endoluminal DeviceThe FRED system is flow diverter with a compliantclosed-cell paired stent (aka ‘stent within a stent’) com-posed of single wire braid self-expanding nickel titan-ium. Initial experience with this device remains limited.Diaz et al40 reviewed 13 patients with 14 treated aneur-ysms. Although no long-term angiographic data areavailable, there were no technical or immediate postpro-cedural complications. The authors concluded thedevice was technically easy to deploy and articulates theability for the device to be recaptured after partialdeployment and to maintain its internal shape in tortu-ous vessels as particular advantage. Long-term clinicaland angiographic clinical studies are necessary tofurther study this device.

p64The p64 Flow Modulation Device is a braided mesh tubecomposed of a 64 nickel–titanium nitinol alloy. Thedevice is compatible with a 0.027″ ID microcatheter. It isavailable in sizes 2.5–5 mm diameter with 12–36 mmlength. There are limited clinical data on this flow diver-ter, with an initial experience and technical aspectsreported by Briganti and colleagues. In their series ofsix intracranial aneurysms, immediate post-treatmentangiography showed reduced flow into all aneurysms,although no long-term angiographic data are available.41

The authors reported no periprocedural technical com-plications and no early or delayed aneurysm rupture, noischaemic or haemorrhagic complications, and no

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neurological morbidity or deaths. The authors note thatthe mechanical detachment feature, with 100% retriev-ability, is a potential advantage of this new device.41

Tubridge flow diverterThe Tubridge is a flow diverter developed by MicroPortMedical Company and is a braided, self-expandingstent-like device with flared ends. A large Tubridge(diameter ≥3.5 mm) is braided with 62 nickel–titaniummicrofilaments and 2 platinum–iridium radiopaquemicrofilaments, whereas a smaller Tubridge (diameter<3.5 mm) is composed of 46 nitinol and 2 platinum–

iridium microfilaments. Initial experience from Zhouet al42 on 28 patients with large or giant ICA aneurysmsshowed a 0% morbidity and mortality rate periprocedu-rally. At a mean follow-up of 10 months, 72% of theaneurysms were completely occluded and 24% wereimproved, with 4% showing no significant change.42

After approval from the Chinese FDA, the device is cur-rently being studied in a multicentre, randomised, con-trolled clinical trial.43 In this study, Zhou et al will enrol124 patients and randomise into a treatment group con-sisting of Tubridge or Tubridge with coils versus acontrol group treated with stent-assisted coiling.

INTRASACCULAR FLOW DISRUPTION AND THE WOVENENDOBRIDGE DEVICEFlow diversion has proven promising to overcome limita-tions of traditional endovascular coiling techniques forthe treatment of sidewall aneurysms; however, themanagement of large bifurcation aneurysms remains achallenge. For these bifurcation aneurysms, coiling with

adjunctive devices such as temporary balloon protectionor stent assistance are associated with increased proced-ural complexity, which lead to a higher chance ofcomplications. Additionally, use of stents requiresperiprocedural dual-antiplatelet therapy with inherentrisks of haemorrhage. In this context, a new device wasdeveloped to provide an innovative alternative strategyvia intrasaccular flow disruption called the WovenEndoBridge (WEB; Sequent Medical, Aliso Viejo,California, USA). The WEB is a self-expanding, oblate,braided mesh of nitinol wires that is deployed into theaneurysm sac itself (figure 3). The initial WEB deviceconsisted of a dual layer (WEB DL) design of inner andouter braids; however, this has since evolved to a singlelayer (WEB SL) device with a higher number of nitinolwires providing similar flow disruption effects.44 Prior todetachment of the WEB device, it can be fully retrieved.Additional modifications of the WEB SL device include aspherical shape (WEB SLS) and enhanced visualisation(WEB EV) provided by platinum-cored nitinol wires.Once a WEB device is deployed within the aneurysm sac,the WEB modifies the blood flow at the aneurysm neck,which induces thrombosis within the aneurysm. Figure 4demonstrates two case examples of middle cerebral artery(MCA) bifurcation aneurysms treated with the WEBdevice.Several retrospective institutional studies in Europe

have demonstrated the initial safety and efficacy ofthe WEB device.44–47 Given the promising data, twoprospective, good clinical practice series were con-ducted in Europe: WEBCAST and French Observatory.These single-arm, prospective, multicentre studiesincluded ruptured and unruptured bifurcation

Figure 3 WEB intrasaccular flow disruption system. SL, single layer; WEB, Woven EndoBridge; WEB SLS, WEB SL device with

a spherical shape. Images provided by Sequent Medical.

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aneurysms located in the basilar artery, middle cere-bral artery, anterior communicating artery and ICAterminus.48 Postoperative, 6-month (in WEBCAST)and 1-year aneurysm occlusion was independentlyevaluated with a three-grade scale: complete occlu-sion, neck remnant and aneurysm remnant. A total of113 patients with 114 aneurysms were treated. Therewas no mortality at 1 month, and morbidity was 2.7%.At 1 year, complete aneurysm occlusion was observedin 56.0%, neck remnant in 26.0% and aneurysmremnant in 18.0%. Worsening of aneurysm occlusionbetween the procedure and 12 months was observedin 2.0% and between 6 months and 1 year in 7.1%.48

The combination of these two prospective studies repre-sents the largest series of WEB cases in the literature. Theadvantage of the WEB device is the combination of anendosaccular approach with flow diversion. This elimi-nates the need for dual antiplatelet therapy required forendoluminal devices and subsequently allows use in theruptured aneurysm setting with subarachnoid haemor-rhages. Although the data are encouraging, the availabil-ity of the WEB device is still limited primarily to Europeand South America. In the USA, the multicentre clinicaltrial for premarket approval of the WEB completed enrol-ment in November 2015 and the data from the trial arestill being collected.

NEUROENDOVASCULAR CATHETER ACCESS SYSTEMAs neurointervention evolves with innovative devicessuch as flow diverters and intrasaccular devices, catheter

access systems have also become more sophisticated.Fundamental to any neurointervention is the need for astable catheter access and delivery system that offersvarying degrees of trackability, distal and proximalsupport, and precise distal targeting. In essence,increased guide catheter support is required for proce-dures with larger device delivery systems, tortuousanatomy and distal targets.Classically, neurointerventions were performed using a

biaxial system consisting of a relatively rigid guide cath-eter (eg, Envoy) positioned in the cervical ICA and asmall flexible microcatheter that was advanced intracra-nially to the target of interest. Often the microcatheterhas to be advanced a considerable distance from thesupporting guide catheter, thereby reducing the controland tactile feedback for the operator and increasing thepropensity for unwanted slack in the system. These lim-itations are amplified in older patients with significantvessel tortuosity, and it can lead to technical failures thatnecessitate alternative more invasive approaches such asdirect carotid puncture.There has been a paradigm shift in the design and

approach to catheter support systems for cases of flowdiversion from a classic biaxial set-up with cervical posi-tioning of guide catheters to a more robust triaxialsystem with intracranial positioning of intermediatesupport catheters. This is secondary to the comparativelylarger size of the flow diverter delivery microcatheter(0.027″ ID for the PED) and the significant intradeploy-ment manipulations required for proper device implant-ation. The Navien (Medtronic Neurovascular, Irvine,

Figure 4 WEB treatment of MCA bifurcation aneurysms. Top row is a small left MCA bifurcation aneurysm. Bottom row is a

large right MCA bifurcation aneurysm. WEB, Woven EndoBridge. Images provided by Sequent Medical.

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California, USA) is a newer generation 5 French (0.058″ID) distal intracranial catheter that is highly trackableand atraumatic with a large bore lumen that can accom-modate 0.027″ ID delivery microcatheters used for flowdiverters with added room for flush and contrast injec-tions. There has been significant experience with theNavien for use in a variety of endovascular aneurysmtreatments, including PED as well as stent-assistedcoiling. These experiences demonstrate successful track-ability of the catheter consistently to intracranial loca-tions such as the ICA and MCA despite vesseltortuosity.49 50 The Navien catheter has also been instru-mental when used as a salvage technique for caseswhere PED opening fails with standard deploymentmanoeuvres.51

This paradigm shift towards more robust multiaxialcatheter support systems has ushered the developmentof additional new intermediate support catheters similarto the Navien catheter. One such example is the Catalystclass of intermediate catheters (Stryker Neurovascular,Freemont, California, USA). The 5 French Catalyst hasdemonstrated significant advantages of enhanced track-ability with reliable stable intracranial support for deliv-ery of modern neuroendovascular devices, includingflow diverters such as the PED Flex and Surpass, as wellas the intrasaccular devices such as the WEB (seniorauthors’ experience, unpublished data). The import-ance of stable catheter support systems translates toimproved safety and ease for sophisticated neurointer-ventional treatments.

PENDING NEW TECHNOLOGYAn array of potential new technology is on the horizonfor the neurointerventional field. These include coatedflow diverters for reduced thrombogenicity, self-expanding three-dimensional mesh endosaccular devicecalled the Medina (Medtronic Neurovascular, Irvine,California, USA), smaller profile WEB devices to addressbroader range of aneurysm sizes (figure 5), and an add-itional new intrasaccular flow disrupter called the Artisdevice (Medtronic Neurovascular, Irvine, California,USA). These new technologies will continue to expandthe neuroendovascular tools available to treat a widevariety of aneurysm types with improved safety and

efficacy. It is fair to say the management of intracranialaneurysms is constantly evolving, with sophisticated tech-nology at the forefront of innovation.

Contributors BJ drafted the manuscript. MP provided illustration of the flowdiversion concept. GPC and ALC assisted in critically revising the manuscript.L-ML conceived of the manuscript and critically reviewed the importantintellectual content. All authors read and approved the final manuscript.

Competing interests ALC is a proctor for the Pipeline Embolization Device(Medtronic Neurovascular, Irvine, California, USA) and a consultant forMedtronic; a proctor for the Surpass device (Stryker Neurovascular, Fremont,California, USA) and a consultant for Stryker Neurovascular; a proctor for theFlow-Redirection Endoluminal Device (FRED) device (Microvention, Tustin,California, USA) and a consultant for Microvention; a proctor for the WovenEndoBridge (WEB) device (Sequent Medical, Aliso Viejo, California, USA) anda consultant for Sequent. GPC is a consultant for Medtronic Neurovascularand Microvention.

Provenance and peer review Commissioned; externally peer reviewed.

Data sharing statement No additional data are available.

Open Access This is an Open Access article distributed in accordance withthe Creative Commons Attribution Non Commercial (CC BY-NC 4.0) license,which permits others to distribute, remix, adapt, build upon this work non-commercially, and license their derivative works on different terms, providedthe original work is properly cited and the use is non-commercial. See: http://creativecommons.org/licenses/by-nc/4.0/

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Figure 5 Optimisation of WEB

platform. WEB, Woven

EndoBridge. Images provided by

Sequent Medical.

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